LU Sijia, REN Yu, WANG Xu, SI Mingming, FAN Yuchi

(State Key Laboratory of Advanced Fiber Materials, College of Materials Science and Engineering, Donghua University, 

Shanghai 201620, China)

Extended abstract:

[Background and purposes] Cold sintering process (CSP) enables the densification of ceramic materials at temperatures far below their melting points (typically below 1/3T_m) through the introduction of an intermediate liquid phase, which plays a critical role in facilitating particle rearrangement, material dissolution and mass transportation. Although cold sintering technology has been successfully applied to low-temperature sintering of hundreds of oxide ceramics, transparent ceramics, semiconductor materials and even ionic compounds, it is necessary to determine the appropriate intermediate liquid phase required for cold sintering through a limited number of experiments, as there is little mechanistic guidance on how to screen for it. It is widely accepted that the intermediate liquid phase is a homogeneous medium solely responsible for dissolving ceramic particles and promoting material transportation, while understanding of the chemical nature of the intermediate liquid phase remains limited. Inspired by the deposition process of geology, the intermediate liquid of cation rich in anions or cation rich in cations may have a significant impact on the cold sintering densification of ceramic materials. Calcium fluoride (CaF₂) transparent ceramics, known for their excellent optical transparency and broad transmission range, are typically sintered at temperatures exceeding 1000 ℃, when using conventional methods, leading to high energy consumption, grain coarsening and volatilization issues. In this study, we aim to systematically compare the effects of liquid phases [deionized water, KF solution (anion-rich) and CaCl₂ solution (cation-rich)], as well as dry pressing without any liquid. It is demonstrated that cation-rich liquid phases significantly enhance the dissolution-precipitation process, enabling low-temperature fabrication of highly transparent CaF₂ ceramics, with outstanding optical and mechanical properties.

[Methods] CaF₂ nano-powders were synthesized via a co-precipitation method using Ca(NO₃)₂·4H₂O and KF·2H₂O as starting materials. The precipitants were vacuum-dried at 60 ℃ for 24 h to obtain CaF₂ nano-powder. The obtained powders were mixed with 10 wt.% of intermediate liquid phase, as well as dry pressed without any liquid, followed by even grinding for 5 min. The mixtures were sintered at 300 ℃ and 500 MPa for 1 h. Relative density was measured by using the Archimedes method. Phase composition and crystal structure were characterized by using X-ray diffraction (XRD). Microstructural evolution and grain boundary features were observed by using field-emission scanning electron microscopy (FE-SEM) and aberration-corrected transmission electron microscopy (JEM-ARM 300). In-situ sintering shrinkage curves were recorded to monitor the densification behavior. Optical transmittance was measured by using a spectrophotometer (Lambda 950, PerkinElmer). Compressive strength (SHIMADZU AGX-5kN Japan) and Vickers hardness were used to evaluate mechanical performances. Grain growth kinetics were analyzed by fitting grain size at different temperatures and holding times to calculate the activation energy of sintering.

[Results] Samples processed with CaCl₂ solution (cation-rich) achieved relative density of ≥99% after sintering at 300 ℃ and 500 MPa for 1 h, far superior to those processed with KF solution (anion-rich) (94%), deionized water (~85%) and absence of liquid (~80%). Samples processed with CaCl₂ solution (cation-rich) exhibit uniform grain size (~190 nm), together with clean grain boundaries and pore-free microstructures. KF-assisted sintering (anion-rich) led to uniform grain size (~171 nm), with inhomogeneous grain growth and residual porosity at early holding stage. TEM imaging further proved the presence of atomically sharp clean grain boundaries in the CaCl₂-assisted sintered samples, which are essential for high optical transparency. In-situ shrinkage analysis results revealed that the rapid densification of the CaCl₂-assisted sintered samples occurred predominantly during the heating stage, whereas KF-assisted sintered samples exhibited delayed densification mainly during the holding time stage. This is ascribed to the enhanced densification with CaCl₂, which is attributed to the rapid supersaturation of Ca²⁺ ions in the liquid phase during heating, accelerating CaF₂ precipitation and establishing a dynamic dissolution-precipitation equilibrium. In contrast, the excess F- ions resulted in the ionic driving force for the precipitation of CaF₂, making non-uniform precipitation to occur and hence leading to significant differences in grain size, more pores and relatively low density. With extended holding time and the combined effect of uniaxial pressure, more Ca²⁺ and F⁻ ions are dissolved, while the liquid phase tends to be supersaturated, thereby initiating the dissolution-precipitation process. Microstructural observation results showed that the CaCl₂-assisted sintering activation energy for grain growth was 38.74 kJ mol⁻¹, significantly lower than that for KF-assisted sintering (42.2 kJ·mol⁻¹) and nearly an order of magnitude lower than conventional sintering (~300 kJ·mol⁻¹). The resultant CaF₂ ceramics sintered with CaCl₂ exhibit a transmittance of 80% at 700 nm, compressive strength of (226.06±5.22) MPa and Vickers hardness of (2.03±0.10) GPa, outperforming samples prepared with other liquid phases.

[Conclusions] The optimal sintering conditions included sintering temperature of 300 ℃, pressure of 500 MPa and holding time of 1 h, with which dense CaF₂ ceramics with a relative density of 99% were obtained. It is demonstrated that the cation-rich liquid phases-CaCl₂ solution could be used to significantly enhance the cold sintering densification of CaF₂ transparent ceramics by promoting Ca²⁺ rapid supersaturation and accelerating the dissolution-precipitation process. The use of CaCl₂ as an intermediate liquid phase enables the fabrication of highly transparent ceramics with clean grain boundaries and uniform microstructure. The CaCl₂ solution (cation-rich) sintering activation energy is reduced to 38.74 kJ·mol⁻¹, further reducing the energy barrier required for atomic diffusion. The resultant ceramics exhibit excellent optical transmittance (80% at 700 nm) and mechanical properties [compressive strength of (226.06±5.22) MPa and Vicker’s hardness of (2.03±0.10) GPa]. This discovery deepens the understanding of the role of the intermediate liquid phase in the preparation of transparent ceramics by cold sintering from the perspective of ion interaction mechanisms, providing a new approach for the selection of intermediate liquid phases for other functional ceramics.

Key words: cold sintering; intermediate liquid phase; dissolution-precipitation; transmittance; calcium fluoride transparent ceramics